In recent years, there has been an increasing inclination towards energy saving concerning industrial products, from the viewpoints of coping with environmental problems and the rising crude-oil prices. This trend is also seen in the field of working vehicles (for example, construction vehicles and industrial vehicles) hitherto dependent mainly on hydraulic drive systems powered by diesel engines. In this field, there have been an increasing number of cases of contriving higher efficiency and energy saving by electric-motorization.
For example, in the case where a drive part of a working vehicle is electric-motorized (namely, where an electric motor is used as a drive source), many energy-saving effects can be expected, such as high-efficiency driving of an engine (in the case of a hybrid model with an engine mounted thereon), enhancement of power transmission efficiency, and recovery of regenerative electric power as well as a reduction in the amount of exhaust gas. In the field of working vehicles, fork lifts have been electric-motorized markedly, and the so-called “battery fork lifts” in which a motor is driven by electric power from a battery have been put to practical use. Besides, recently, in engine-type hydraulic excavators and fork lifts and the like, “hybrid vehicles” in which a combination of diesel engine with electric motor is used as a drive source have started to appear as products. Further, the working vehicles that are expected to exhibit a fuel consumption-reducing effect when hybridized include the wheel loader. A conventional wheel loader is a working vehicle that excavates and transport earth and sand or the like by a bucket part of a hydraulic working device mounted to a front side of the vehicle while traveling by transmitting power of an engine to wheels through a torque converter and a transmission (T/M), for example.
Meanwhile, as a typical working mode of the wheel loader, there is a V-shape excavating work. In the V-shape excavating work, the wheel loader first travels forward towards an object of excavation such as gravel heap, plunges into the object of excavation, and then crowds (tilts) the bucket to load the matter to be transported, such as the gravel, into the bucket. Thereafter, the wheel loader travels backward to return to an original position, and travels forward towards a transport vehicle such as a dump track while raising the bucket. Then, the wheel loader dumps the bucket to load (dump) the matter to be transported onto the transport vehicle, and thereafter travels again backward to return into the original position. The vehicle repeats this work while moving along the V-shaped locus as described above.
In such a V-shape excavating work, the wheel loader performs the work by distributing the power of the engine to a traveling part and a hydraulic part of a hydraulic working device. The V-shape excavating work is basically a work conducted on a substantially flat ground surface. In this case, the wheel loader plunges into the object of excavation with its hydraulic working device, and scoops up a large amount of the excavated matter with the hydraulic working device while generating a great traction force; therefore, a high power is needed for this motion.
On the other hand, traveling motions during such an excavating work may be conducted at a comparatively high vehicle velocity, from the viewpoint of contriving enhancement of operating efficiency (working efficiency) per time. In such a case, the vehicle velocity is largely lowered at the moment the wheel loader plunges into the object of excavation, and there is a possibility that variations in the engine revolution speed may fail to follow up to variations in vehicle velocity, and the engine revolution speed may not yet be sufficiently raised at the time when power is needed for the excavating work. In a state where the engine revolution speed has not yet been sufficiently raised, power necessary for excavation cannot be drawn from the engine. Therefore, there arises a possibility that the object of excavation cannot be excavated sufficiently by the bucket or that the earth and sand cannot be smoothly scooped up by the bucket. If such a situation is generated, the excavating work is redone after the engine revolution speed is raised to a required revolution speed. As a result, the operating efficiency is lowered.
A method of solving such kind of power deficiency is described, for example, in JP-2008-008183-A. In this technology, an auto-operated wheel loader is operated in such a manner that when a state of approaching an object of excavation is detected beforehand, the engine revolution speed is raised, so as to start an excavating work with the engine revolution speed at a high value, thereby contriving to avoid deficiency in power from the engine and to achieve smooth transition to the excavating work.